Accurate measurement of wind turbulence is important in any meteorological study where the stability of the atmosphere must be determined. Atmospheric turbulence is the irregular fluctuation of horizontal and vertical wind currents, and is primarily the result of thermal and mechanical forces on the air parcels, thereby producing turbulent motions or eddies of varying sizes. A few areas where the measurement of turbulence has become critically important include pollutant dispersion studies, long-range weather forecasting, wind energy siting, wind shear warning, emergency response wind monitoring, sound transmission analyses, microwave communications assessments and aircraft vortex monitoring.
Several classes of instruments exist which can measure wind speed and direction, but unfortunately, also incorporate inherent limitations which have led to a plateau in the breadth and resolution of wind turbulence data available to scientists and meteorologists. The most common instrument for measuring wind speed and direction is also the oldest: the cup and vane anemometer. Originally invented over a hundred years ago, this inexpensive device can be mounted at stationary locations or on slow-moving vehicles and ships, but is only capable of measuring the mean wind speed and direction; meanwhile, small-scale wind gusts remain undetected. As a result, the cup and vane anemometer has proven inadequate for wind turbulence monitoring.
A more recent advancement is the sonic anemometer, which determines the mean wind speed and direction at a location immediately adjacent the instrument. The instrument can also measure atmospheric turbulence, but only with a limited frequency response typically below 20 Hz. The sonic anemometer operates on the principle that the motion of air affects the propagation of ultrasonic sound waves. The instrument uses a proscribed sonic pathway through a test region, typically between 10 and 20 centimeters in length, with a transmitter on one end and a receiver on the other. Several commercial designs employ three pathways orientated at various angles through the same test region to provide real-time three-dimensional measurement of the air velocity. However, the instrument has several drawbacks. The instrument is bulky and heavy, both because of the size of the test region and because the transmitters and receivers are commonly positioned on the tips of long arms in order to be as aerodynamically unobtrusive as possible to avoid inducing turbulence within the measurement region. Because of it's bulk and weight, the application of the sonic anemometer is limited to ground or ship-based operations. As the tallest practical weather tower may be approximately 150 m (500 ft), accurate measurement of air turbulence with a sonic anemometer is limited to the first few hundred feet above ground level, but are typically only used below 30 m.
Three other technologies which can remotely monitor atmospheric turbulence have also appeared over the past few decades, but are considerably more expensive and complicated than cup and vane or sonic anemometers. Doppler LIDAR (Light Detection and Ranging) shoots a narrow beam of light, such as that coming from a laser, into the upper atmosphere, where the light is reflected back by airborne particles or moisture droplets and observed by specialized optics. LIDAR is able to detect fluctuations in wind velocity and direction from miles away, but the cost of the laser, optics, and associated electronics make this system far too expensive for any but the most well-funded scientific installations. Doppler SODAR (Sonic Detection and Ranging) works on similar principles, except that sound is employed instead of light, and the physical process being measured is backscattered sound energy caused by atmospheric turbulence rather than direct reflection. The instrument measures turbulence up to several thousand feet above ground, but can require a reflective parabolic antenna up to four feet in diameter. Doppler wind profiler RADAR (Radio Detection and Ranging) works on a similar principles of the LIDAR and SODAR, although using radio waves. An additional limitation of the RADAR is due to the bandwidth requirements to operate the RADAR and the potential for interference with television and other radio signals. The bulk and fragility of the equipment dictates that LIDAR, SODAR, and RADAR systems are normally installed at permanent ground-based installations.
In summary, the present state of the art in atmospheric turbulence measurement is generally limited to either low-cost systems that determine only the mean wind speed and direction, or to higher-end instruments that measure also turbulence, but are either limited by their size, complexity, or cost to lower altitudes and localized installations.